As children acquire their first language, they learn how to form new words by compounding (e.g., from egg to plate to egg-plate), and by derivation with an affix (e.g., from pull to puller) or without one (e.g., from a sled to to sled). How and when do children learn the rules for word-formation? In earlier research, I proposed and tested six acquisitional principles that dealt with transparency of meaning, simplicity and regularity of form, productivity, conventionality, and contrast in meaning among the elements used for new words. The present research focuses on further predictions from these principles through (a) analyses of an extensive longitudinal corpus of innovative word-forms from one child, (b) cross-sectional studies designed to elicit, judge, and analyze novel words, and (c) cross-linguistic studies to collect cross-sectional data on Hebrew, and to explore the influence of productivity for both English- and Hewbew-speaking adults. These studies will be used to validate the six principles and assess their "weight' in determining order of acquisition.